The present invention relates to a fork unit that can be applied to the fixed front plate member of a lift truck and which can be side-shifted with respect to the said plate and the position of the forks of which can be adjusted with respect to a bearing frame.
There are well known to persons with ordinary skill in the art lift trucks in which the forks or equivalent gripping means are mounted on a fixed front support plate of the truck as a result of the interposition of a device that permits the forks to be shifted sideways. Such a device is commonly known in the art as a side-shift device, and it allows the forks to be shifted so that the taking-up and depositing of the load can be done correctly, even with the lift truck only in approximate alignment with the load.
However, the adoption of a side-shift device on a fork lift truck-entails an unwanted increase in load overhang equal to the thickness of such device, and accordingly increases the overturning moment of the lift truck itself.
For this reason, for a side-shift device to be advantageously mounted on a fork lift truck, its essential pre-requisite is low thickness--in any case a thickness reduced to the indispensable minimum. A fork side-shift device embodied according to the conventional art comprises a guide which is coupled to the truck plate member (with lifting movement) and on which the side-shift device plate (with lateral movement) slides. The distance between the former plate and the latter plate, here expressed as side-shift device thickness, results in an identical increase in fork overhang with respect to the barycentre of the lift truck and thus to a decrease in lift capacity over and above the decrease necessarily caused by the weight of the device per se. Given the foregoing, it will be seen how much importance attaches to providing a construction that will appreciably reduce load overhang.
In the known art, two factors most affect the total side-shift device thickness: the first of these is the thickness of the side-shift device plate and its guides, and the second is the overall bulk of the hydraulic cylinder which, acting between the front plate of the lift truck and the fork side-shift device, controls the side-shift of the latter with respect to the fixed plate.
The hydraulic cylinder must in fact be able to develop a thrust adequate to the load to be moved carried by the lift truck and to the passive resistances due to the friction between the sliding-contact guides between the fixed and movable plates. Moreover, being fed by fluid at the pressures usual in oleodynamic circuit applied to this type of device, the cylinder must be of adequate section.
Manufacturers of lift trucks and relative equipment have long put forward different solutions for achieving an ideal fork side-shift device, i.e. one which keeps weight and thickness and loss of operator load viewability to a minimum. There have been proposed for this purpose fork side-shift devices having rolling guides, which without doubt lower the power required and thus the dimensions of the hydraulic cylinder; but, at the same time, this involves a very significant complexity of the structural elements of the fork side-shift device.
Entirely ball-packed guides have also been proposed German patent 2317758), but again with unsatisfactory results due to deformations of the ball bearing races in the terminal areas of the guides, which cause jamming.
Lastly, guides with inserted shoes, fixed with screws, made of hardened material, have been proposed, but with negative results owing to increases in thickness and costs.
As is evident from the foregoing, every effort has been made to reduce the aforesaid passive resistances so as to be able to decrease the thrust required of the hydraulic operating cylinder, and thus also its diameter, and consequently to reduce the total thickness of the fork side-shift device. A rational disposition and configuration of parts, as illustrated in Italian model application 22923 B/78, makes it possible to have a side-shift device with a total thickness only slightly greater than the side-shift plate itself, such slightly greater thickness being due to the bulk of the members for coupling to the lift truck plate and to the slide surfaces. This solution appears to attain the minimum possible thickness for the purpose of embodying a fork side-shift device to be coupled to the fixed plate of a lift truck.
To further reduce bulk, it has been proposed to eliminate the usual vertical-shift-only fork-bearing plate featured in all lift trucks by placing the fork side-shift device between the slide running in the truck masts and the plate so that this last can shift horizontally and vertically. However, this solution calls for a modification of the structure of the lift truck itself which has to be equipped with the side-shift device already during its construction, and so the said device loses its nature of an additional element to be applied to the standardized plate of a lift truck, if and when necessary, for the handling of certain loads with the use of forks.
The overall object of the present invention, on the other hand, is to solve the problems of the know art in a satisfactory manner by wholly abandoning the traditional structure of a fork side-shift device consisting of a movable fork-bearing plate than can be slidingly coupled to the truck plate with a hydraulic operating cylinder between the two.
The present invention, instead, proposes an operating unit in which the forks are integrated with a side-shift device.